Thanks for your quick reply, Till. You got me pointed in the right direction.
Here's what I did. Please let me know if you see any problems with my approach.
Create a new Apache config file for munin:
Insert the following lines into the file:
Alias /munin /var/www/munin
Allow from all
Create a new folder and give ownership to munin:
chown -R munin:munin /var/www/munin
Edit the munin config file to point to the new folder:
change htmldir to point to /var/www/munin:
Secure /var/www/munin with .htaccess
Now I can access munin from http://www.mytestsite.com/munin